a) Field of the Invention
This invention relates to a new or improved drive cap for use in association with pile hammers or drivers, particularly diesel pile drivers, and also to a combination of the new drive cap with a pile driver.
b) Description of the Prior Art
In conventional pile hammers as shown in FIG. 1, the piston 10 sliding within the vertically oriented cylinder 11 is driven by an explosion of diesel fuel that has been compressed to its ignition point in the combustion chamber 12. An impact block 13 has a head 14 that extends from the lower end of the cylinder, the impact block being slidably received in the cylinder. The head 14 is positioned in contact with an arrangement of cushioning material 15 positioned in a recess on the upper side of a pile cap 16, the lower side of which has a recess to receive the upper end of the pile (not shown). The drive cap is slung below the hammer by cables (not shown) in such a fashion as to restrict the extent of which it can move away from the lower end of the cylinder and thus restrict the maximum extension of the impact block 13 with respect to the cylinder. This degree of movement is determined to be a point that allows the impact block to be displaced under the force of the exploding fuel change to drive the pile, without allowing the impact block to be forced out of the lower end of the cylinder.
The upper end of impact block forms the bottom of the combustion chamber. As the piston 11 falls, fuel is introduced into the combustion chamber 12. The falling ram or piston 11 compresses the fuel air mixture in the combustion chamber and ignites it, driving the piston upwards and the impact block downwards. The impact block accordingly applies a driving force to the head of the pile through the cushioning material 14 and the drive cap 16 to drive the pile into the ground.
There are two fundamental problems that limit the efficiency of existing diesel pile hammer designs. The first of these concerns the cushioning material which has to transfer energy from the impact block 13 to the pile in the driving direction, and also has to attentuate forces generated by a rebound of the pile, so that the hammer is not damaged by this effect, referred to as "wracking" which can result in greatly increased maintenance costs. In delivering energy to the pile, the function of the cushion material is to increase the period over the which the energy is transferred, to avoid damage due to high peak impact loads on the pile head. In other words, the object is to lower the peak forces created by the falling ram, thus protecting the head of the pile and pushing the pile into the ground faster by extending the time period over which the driving force is applied. In reality, however, a substantial quantity of the energy applied to the cushioning material is absorbed and dissipated as heat that first hardens and ultimately causes the material to deteriorate. Thus, the pile is robbed of some of the energy that should be applied to drive it into the ground. Additionally, the cushioning material, particularly as it hardens, does not efficiently prevent transfer of energy back to the hammer from rebound of the pile. The results are a pile that takes longer to be driven, and a hammer that requires excessive maintenance.
The second problem with existing designs concerns the weight of the drive cap. To overcome the destructive impact forces created by the falling piston and the ensuing transferred energy, drive caps are made of durable materials which are very dense. Accordingly, the typical weight of the cushioning material and drive cap amounts to between 1500 and 2000 pounds, and this weight rests entirely on the head of the pile. This weight in addition to the weight of the pile must be mobilized before any driving of the pile is accomplished, so that there is additional energy wasted in overcoming the inertia of the pile cap. Altogether, the combined effect of the problems discussed above is to waste up to 65% of the kinetic energy of the falling ram or piston, while increasing the effective inertia of the pile, and transferring damaging rebound energy back to the pile driver.
An energy transverse system of a pile driver should desirably drive the pile as efficiently as possible while minimizing power driver maintenance. Existing systems do not achieve these conditions.